Experimental Investigation of Bone Conduction Pathways: 3D Surface Motion and Wave Propagation

Objectives: Investigation of bone conduction sound propagation by the osseous and non-osseous pathways and their interaction depending on stimulation site and coupling method of the bone conduction hearing aid (BCHA), studied based on measurements of surface wave patterns.

Methods: Experiments were conducted on five Thiel embalmed whole head cadaver specimens. The electromagnetic actuators from commercial BCHA (Baha® Cordelle and BoneBridge®) were used to provide stepped sine stimulus (0.1-10 kHz). Osseous pathways were sequentially activated by mastoid stimulation via a BI300 implant, Baha® Attract BIM400 transcutaneous magnet and a 5-Newton steel headband. Non-osseous pathways were activated by stimulation on the eye, neck, and dura via a 5-Newton steel headband and compared with stimulation on mastoid or forehead. Motions of skull surface (ipsilateral, contralateral, and the superior skull surfaces) were measured via a three-dimensional laser Doppler vibrometer (3D LDV) system. Around 60 points were measured covering approximately 8×8cm² per area.

Results: 3D LDV data indicates that the skull surface motion undergoes complex spatial motion with similar contributions, at higher frequencies, from all motion components (normal and tangent), under all stimulation modes. Motion of the cadaver heads at low frequencies is rigid-body-like.

Conclusion: Comprehensive experiments, including measurements (scanning) of the 3D motion over large sections (>200cm²) of the skull surface, with various stimulation positions and coupling methods allow for detailed exploration and differentiation of the individual contributions of the various bone conduction pathways.

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